Ignition system for internal combustion engines

ABSTRACT

An ignition system for internal combustion engines comprises an electronic circuit arrangement which under variable operating conditions, such as high and low speeds of the engine, high and low supply voltages, supplies a desired energy to the ignition coil so as to produce ignition sparks having, for example, a constant energy. For this purpose, the ignition coil is connected via a semiconductor switch to the supply source for a given time. The pulse duration is determined by the time constant of an RC network and by the voltage of the supply source and, as the case may be, by the ambient temperature.

United States Patent Korteling Oct. 15, 1974 [54] IGNITION SYSTEM FOR INTERNAL 3,587,552 6/1971 Varaut 123/148 E COMBUSTION ENGINES 3,599,618 8/1971 Schuette 123/148 E 3,605,713 9/1971 LeMasters 123/148 E Inventor: Aart Gerrit Korteling, Emmasingel, 3,666,989 5/1972 Boyer 123/14s E Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New Primary Examiner Laurene Goodridge Y k N Y. Assistant Examiner-Cort Flint Filed: Sept. 1971 1:23:32; Agent, or Firm Frank R. Tr1far1, Bernard [21] Appl. No.: 177,945

[57] ABSTRACT [30] Forelgn Apphcatmn Pnomy Data An ignition system for internal combustion engines Sept. 5, 1970 Netherlands 7013168 omprises an electronic circuit arrangement which under variable operating conditions, such as high and [52] US. Cl 123/148 E, 315/209 T low speeds of the engine, high and low supply volt- [51] Int. Cl. F02p 3/02 ages, supplies a desired energy to the ignition coil 50 [58] Field Of Search .1 123/148 E as to produce ignition sparks having, for example, a constant energy. For this purpose, the ignition coil is References Cited connected via a semiconductor switch to the supply UNITED STATES PATENTS source for a given time. The pulse duration is deter- 3,087,090 4/1963 Konapa 123/148 E mined by the time Constant of an RC "em/01k and by 3,238,416 3/1966 Huntzinger 123/148 E the voltage of the pp Source and, as the Case y 3,322,107 5/1967 Mieras et a1. 123/148 E y th a nt t mp atur 3,473,06l 10/1969 Soehner et a1. 123/148 E 3,575,153 4/1971 Hardin 123/148 E Clams 7 Drawmg Flgules All IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINES The invention relates to an ignition system for internal combustion engines which is to be connected to an ignition coil for producing sparks at ignition electrodes and is provided with a control input to be connected to the contact-breaker system of the internal combustion engine. The ignition system includes a semiconductor switch to be connected between a supply source and the coil and a control circuit connected to the control input and also to the control electrode of the semiconductor switch. The control circuit includes a control pulse generator which on reception of a control signal applies to the semiconductor switch a control pulse of a duration which is determined by a resistancecapacitance network, so that there is stored in the ignition coil a given amount of energy which at the instant of the trailing edge of the control pulse is released to be dissipated in the form of sparks. French Pat. Specification No. 1,183,698 describes an ignition system of this type. This known ignition system has the disadvantage that the pulse duration is constant. Consequently the energy to be dissipated in the form of sparks depends upon the ambient temperature and upon the supply voltage.

The invention obviates this disadvantage. For this purpose, an ignition system of the type described at the beginning of this specification is characterized in that the control pulse generator includes at least one voltage reference element, and the control pulse generator and the ignition coil are connected to the same supply terminal, the duration of the control pulse being substantially proportional to RC 1n V /V V where RC is the time constant of the resistance capacitance network, V is the supply voltage and V is the voltage of the voltage reference element and In is the natural logarithm, so that a desired amount of energy in the coil is adjustable as a function of the supply voltage and of the ambient temperature. A theoretical consideration of the combination comprising the ignition coil and the pulse generator shows that optimum conditions for the energy are adjustable by matching the coil and the pulse duration, i.e., the choice of the components of the control pulse generator. For example, it is found to be unnecessary to design an ignition coil having a small copper resistance to reduce the influence of the temperature coefficient of the winding. The construction of the ignition coil may be cheaper by using a smaller amount of copper and by taking into account the resulting time constant L/R in the pulse control. The advantage of the pulsed control of an ignition coil, namely the reduced size, may now be increased. As is known, it was already possible to reduce the size of the coil, because owing to the pulsed control the heat generation in the coil was reduced to a low value or even to zero, for example, with the engine at standstill and the ignition system switched on. To improve the understanding of the principle on which the invention is based, a theoretical exposition will now be given.

A coil having an inductance L and a circuit resistance R to which a supply voltage V is applied through a switch, will pass a current 1,, which is determined by:

If pulsed control is used and the time t is determined by a time constant RC multiplied by the natural logarithm of a function F to be defined hereinafter, the exponent in the formula (1) will be:

RCR /L In F and the 6 -power will be:

where B RC'R /L If B 1, formula (1) is simplified t0 If F V /V V where V is the voltage of a reference element, we have:

IL VB/RL VB VB R/ B R/ L The energy stored in the coil is:

it L If Formula (5) shows that this energy is independent of the supply voltage.

If B is not equal to 1, the current is L VB/RLI 1- (VB VR/VB)B] Thus a desired voltage dependence upon the current and hence upon the energy in the coil is obtainable.

If the energy in the coil is to be constant, irrespective of variations in the temperature and the supply voltage, and if R is temperature-dependent, the reference voltage V may be given an equal temperature dependence, the RC combination being inversely proportional about B 1.

The above considerations may be applied to an ignition coil which supplies the high tension for the ignition of the gas mixture in a motor-car engine. In this case the operating conditions generally vary greatly. The temperature may show a difference of, for example, 100C and the supply voltage may vary by a factor of 2. Thus, at a low ambient temperature and with a poor battery only one half of the rated supply voltage and hence only one quarter of the rated spark energy is available. Under these very conditions, which may be combined with damp surroundings or fouled spark plugs, it is important that a fat spark should be available.

erence elements provide a pulse duration which satisfies the relation t RC ln V /V V A particular advantage of the ignition system according to the invention is that its very small size and very slight heat dissipation enable the entire system to be constructed as a unit together with the ignition coil or the contact-breaker system, and in the latter case it may be mounted in the contact-breaker housing which already accomodates the conventional mechanical contact-breaker or an electronic contact-breaker.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a block-schematic circuit diagram of an ignition system using pulsed control,

FIG. 2 is a circuit diagram of an ignition system according to the invention using one transistor,

FIG. 3 is another embodiment of such a system using one transistor,

FIG. 4 is an alternative embodiment of the system of FIG. 3,

FIG. 5 shows an embodiment using a Schmitt trigger,

FIG. 6 shows an embodiment using two voltage reference elements and a monostable circuit, and

FIG. 7 is a graph in which the ignition coil energy is plotted as a function of the supply voltage.

Referring now to FIG. 1, there is schematically shown an internal combustion engine 1 provided with four spark plugs which each have ignition electrodes 2a and 2b between which sparks must be produced at the correct instants to fire a combustible gas mixture in the combustion spaces of the engine. These correct instants and the selection of the spark plugs are controlled from the crankshaft of the engine via a contactbreaker system 3 and a distributor 4.

Because, for example, the contact between a lamination 8 and a lamination 9 is broken in given positions of the contact-breaker system 3, in a control circuit 5 a control pulse generator, which may be fed from a supply source V generates a control pulse which closes the switch 6 for the duration of the control pulse. As a result, a current supplied by a source V' which may be identical with the supply source V will flow in an ignition coil 7. The current is determined by the inductance L of the coil, the resistance R of the coil circuit and by the supply voltage V,,. On termination of the pulse an energy V2 LI has been stored in the coil, where I is the current which flows at this instant. Since the switch 6 will now open, this energy is transferred to the secondary, high-voltage side of the coil and is supplied via the distributor 4 to one of the spark plugs.

FIG. 2 shows a control circuit 5 having a switch 6 and an ignition coil 7 operated by a contact-breaker 3.In this embodiment the switch 6 comprises two transistors 10 and 11 connected in a Darlington configuration.

The control circuit 5 incldues a control pulse generator according to the invention, which comprises a resistor 12, a capacitor 13, a transistor 14 and a collector resistor 15 connected in series with a Zener diode Z. An inverter stage comprising a transistor 16 having a collector resistor 17 amplifies the pulsatory voltage at the collector of the transistor 14 and this amplified and phase-inverted voltage at the collector of the transistor 16 controls the switch 6 at the base of the transistor 10. If the contact-breaker 3 is in the closed condition, the capacitor 13 is discharged through a resistor 18 and the transistor 14 is cut off.

As a result, the transistor 16 is conducting and saturated so that the transistors 10 and l l are cut off. When the contact-breaker 3 is opened, a current flows through the resistor 12, the base-emitter junction of the transistor 14 and the uncharged capacitor 13.

This current is reduced according to a power e by the charging of the capacitor 13. For a certain period of time the transistor 14 is adjusted in its saturation range by this current so that the transistor 16 is cut off and current is supplied to the base of the transistor 10 through the resistor 17. Consequently, the transistors 10 and 11 are substantially saturated, and the supply voltage is applied to the ignition coil 7.

As soon as the base current of the transistor 14 becomes too small to maintain the transistor 14 saturated, the transistor 16 will draw current and the transistors 10 and 11 will be cut off. The resulting pulse duration is so short that the contact-breaker 3 will always close subsequently.

The pulse duration obtained by means of this circuit is approximately equal to:

where 04' is the current amplification factor of the transistor 14 at which this transistor comes out of the saturated condition, 1n is the natural logarithm and V is the Zener voltage of the Zener diode Z. Ensuring that 01' R =R, provides a pulse duration which, according to the invention, is most suitable for controlling an ignition coil.

FIG. 3 shows a circuit arrangement which also satisties the formula (8). A zener diode 35 is included in the base circuit of the transistor 14 the emitter of which is connected to ground and the collector circuit of which includes the resistor 15. After the contact-breaker 3 has opened, the capacitor 13 may be charged through the resistor 12 and the base of the transistor 10 of the switch 6' is no longer fixed with respect to ground through a diode 36. The switch 6 will pass current because base current is supplied to the transistor 10 through the collector resistor 15. When the voltage across the capacitor 13 exceeds the Zener voltage of the Zener diode 35, the transistor 14 will pass base current and be saturated, with the result that the switch 6 is rendered non-conductive. The Zener diode 35 may alternatively be included in the emitter circuit of the transistor 14, as is shown in FIG. 4. In this case a Zener diode 37 must be connected between the collector of the transistor 14 and the base of the transistor 10 to take up a direct-voltage difference, rapid switching being maintained.

In the system shown in FIG. 4 the contact-breaker 3 is connected across the Zener diode 35 so that in the closed condition of the contact-breaker 3, the capacitor 13 is in the substantially discharged condition and the transistor 14 is in saturation. When the contactbreaker opens the capacitor 13 is charged through the resistor 12, while the transistor 14 is cut ofi, so that the switch 6 is controlled through the resistor 15 and the Zener diode 37. When the voltage across the capacitor 13 reaches the value of the Zener voltage of the diode 35 increased by the V of the transistor 14, this transistor is saturated again and the transistors and 11 of the switch 6 are cut off. This occurs because the Zener voltage of the Zener diode 37 has been made greater than the Zener voltage of the Zener diode 35 increased by the saturation voltage of the transistor 14. In FIG. 4 a possible control of the pulse generator 5 is shown in broken lines, the control circuit comprising a contact-breaker 3', a discharge resistor 31 and a gate diode 30 to cut off the switch 6.

In FIG. 5 the pulse generator 5 comprises a multivibrator circuit of the type frequently referred to as a Schmitt trigger. The capacitor 13 is discharged in the closed position of the contact-breaker 3 and is charged through the resistor 12 when the contact-breaker is open. When the capacitor voltage reaches the discrimination level of the Schmitt trigger, the latter changes state so that a steep switch-off edge for the switch 6 is obtained. Transistors 14 and 19 together constitute the Schmitt trigger. Their emitters are interconnected and connected to ground through the Zener diode 35. This Zener diode determines the discrimination level and is the voltage reference element. The base of the transistor 14 is the input of the Schmitt trigger and is connected to the junction point of the resistor 12 and the capacitor 13, which are connected in series between the supply terminal +V and ground. The voltage set up across the collector resistor is applied through a voltage divider comprising resistors 22 and 23 to the base of the transistor 19, which has a collector resistor 21 across which the output voltage of the Schmitt trigger is set up. In the non-operative condition this voltage is zero so that a transistor 25, the emitter of which is connected to the supply terminal V and the base of which is connected to the collector of the transistor 19 through a resistor 24, will be cut off. No current flows in the collector circuit of the transistor 25 with the result that the transistor 28, which constitutes the switch 6, also passes no current.

By connecting the contact-breaker 3 in parallel with the Zener diode 35, in the closed position of the contact-breaker the capacitor 13 will discharge via the base-emitter junction of the transistor 14 so that this transistor is saturated and subsequently will remain saturated owing to the base current supplied through the resistor 12. After the discharge, the collector of the transistor 14 will be at a potential which is a few tenth parts of a volt above earth ground level so that, via the potential divider 22, 23, too low a voltage is applied to the base of the transistor 19, with the result that this transistor is cut off and does not build up a voltage across the collector resistor 21. When the contactbreaker 3 opens, the Schmitt trigger adjusts itself while the transistor 14 is cut off and the transistor 19 is conducting. As a result, a voltage is also set up across the resistor 21 so that the transistor 25 passes current and is saturated, with the result that base current is supplied to the transistor 28. Thus, current will flow through the ignition coil 7. The capacitor 13 is charged via the resistor 12 to the discrimination level, after which the transistor 14 will pass current and and the transistor 19 will be cut off. As a result the transistor 25 and 28 will also be cut off so that the pulsed energization of the coil 7 is terminated. If now the contact-breaker 3 is closed again, there will be no change in the conductivity conditions of these transistors. The Figure shows in broken lines an alternative method of coupling the contactbreaker 3 to the Schmitt trigger. In this alternative embodiment, the bases of the two transistors are connected to ground through diodes 29 and 30 in the closed position of the circuit-breaker 3'.

In FIG. 6 the control pulse generator 5 is a monostable multivibrator circuit comprising the transistor 14 with grounded emitter, collector resistor 15 and base resistor 12, and a transistor 32 with grounded emitter, collector resistor 34 and base resistor 33. The capacitor 13 is connected between the collector of the transistor 32 and the base of the transistor 14. The contactbreaker system 3 is connected between the base and the emitter of the transistor 32, while the switch 6 comprising the Darlington pair 10 and 11 is controlled by the voltage across the resistor 15.

In the cut-off condition of the transistor 32 the charging voltage of the capacitor 13 is limited by a Zener diode 38 serving as a first voltage reference element, while a second voltage reference element in the form of a Zener diode 39 is connected in series with the base resistor 12. By so choosing the Zener voltages that they are equal to one another: V V we again have:

t R C 1 V /V V2 and I Vz VR with B l, which shows that in the time formula V has the greater influence and that V influences the ignition coil current. This may be of importance for any temperature compensations which may be required, as has been mentioned in the above theoretical dissertation.

To illustrate a simple embodiment FIG. 7 is a graph in which the magnetic energy is plotted against the supply voltage, R, C and L being constant and not temperature-dependent. It is assumed that R,, varies by a factor of 1.4 in a temperature range of 100C. The various parameters are standardized: thus, it is assumed that the supply voltage V has a rated value of l and is 0.6 under adverse conditions for a car battery and has a maximum value of 1.4 for a battery which is being charged.

The cuves a and b show the variation of the energy at a low and at a high temperature, respectively, for a conventional system using an ignition coil which is directly operated by a contact-breaker. Curves c, d and e show this variation when using an ignition system according to the invention under the aforementioned conditions. The curves c and d show very clearly the great advantage of the ignition system according to the invention as compared with the curves a and b of a conventional system. In this manner the dimensions can be shosen so that at a high temperature the curve d is approximately obtained, while at a low temperature the curve c is followed. In the case of pulsed control with a constant pulse duration the e-power in formula 1 is significant and the curve a of FIG. 7 follows a lower path than that shown, while the curve b is used as a reference and hence remains unchanged. In the case of expensive proportioning of such a system with a small R i.e., with a high amount of copper, the starting range of the exponential power may be chosen. In this case R disappears from the formulae and there is no temperature-dependence. The curve b, for example, shows the temperature-dependence at any temperature. The graph again shows clearly the considerable improvement due to the steps according to the invention.

what is claimed is:

1. An ignition system for an internal combustion engine comprising, a source of DC voltage, and ignition coil having a positive temperature coefficient of resistance, a semiconductor switching device connected in series with the coil across the terminals of the DC voltage source, a control pulse generator connected to said DC supply terminals and having an output terminal coupled to a control electrode of the switching device for supplying thereto a switching control pulse for periodically switching the switching device on and off, means including the engine contact-breaker for supplying a control signal to said control pulse generator in timed relation to the engine, said pulse generator being responsive to the control signal to generate said control pulse and including an RC network that determines the time duration of the control pulse, a first voltage reference element connected in said pulse generator so as to cooperate with the RC network and the supply voltage to determine the time duration of the control pulse whereby the time duration of the control pulse varies inversely to the DC supply voltage V the value V of the reference voltage of the reference element being chosen relative to the value of the DC supply voltage so that, in cooperation with the RC time constant of the RC network, the time duration of the control pulse is substantially proportional to RC 1n V V V and varies over the non-linear portion of the exponential curve.

2. A ignition system as claimed in claim 1, characterized in that the RC time constant is equal to the time constant L/R of the ignition coil circuit, where L is the inductance of the ingition coil and R is the resistance in the ignition coil circuit.

3. An ignition system as claimed in claim 1 wherein the control pulse generator comprises a Schmitt trigger which includes a first transistor and a second transistor, said RC network comprising the series combination of a resistor and a capacitor connected between the supply terminals of the Schmitt trigger, means connecting the base of the first transistor to the junction point of the resistor and the capacitor, and the collector circuit of the second transistor, which is resistively coupled to the first transistor, delivers the control pulse to the switching device, and means connecting the voltage reference element in the common emitter circuit of the two transistors.

4. An ignition system as claimed in claim 1 wherein the control pulse generator comprises a monostable multivibrator circuit in which the resistancecapacitance network is is connected so that the charging voltage of the capacitor C is equal to the voltage of the first voltage reference element, and a discharge circuit for the capacitor comprising a resistor R conncected in series with a second voltage reference element, the reference voltages of the two reference elements being equal.

5. An ignition system as claimed in claim 1 wherein the control pulse generator includes a transistor connected in common emitter configuration the collector circuit of which includes a collector resistor connected in series with the voltage reference element and the base circuit of which includes said RC network comprising a series combination of a base resistor and a capacitor, which determines the RC time constant, and means coupling the transistor collector circuit to the switching device for delivering the control pulse thereto upon reception of a control signal at the control pulse generator input, said control pulse generator input being formed by the junction point of the base resistor and the capacitor, the base resistor being equal to the collector resistor multiplied by the current amplification factor of the transistor.

6. An ignition system as claimed in claim 1 wherein said pulse generator comprises a transistor with the voltage reference element connected in the collector circuit and the RC netork connected in the base circuit.

7. An ignition system as claimed in claim 1 wherein said pulse generator comprises a transistor with its collector coupled to the control electrode of the switching device, said RC network comprising a resistor and capacitor serially connected across the DC supply terminals and the voltage reference element being connected between the junction of the resistor and capacitor and the base of the transistor, and a diode connected between the control electrode of the switching device and said junction and poled to conduct current towards the junction.

8. An ignition system as claimed in claim 1 wherein said pulse generator comprises a transistor with its collector coupled to the control electrode of the switching device via a second voltage reference element, said RC network comprising a resistor and capacitor serially connected across the DC supply terminals and the first voltage reference element being connected in the emitter circuit of the transistor.

9. An ignition system as claimed in claim 1 wherein said pulse generator comprises first and second transistors connected to form a monostable multivibrator circuit with the capacitor of the RC network connected between the collector of the first transistor and the base of the second transistor, said first voltage reference element being connected to the collector of the first transistor so that the capacitor voltage is determined by the voltage of the first reference element, and a second voltage reference element coupled to the base circuit of the second transistor and as a part of a discharge circuit for the capacitor, the collector of the second transistor being coupled to the control electrode of the switching device for supplying said control pulse thereto.

10. An ignition system for an internal combustion engine comprising, a source of DC voltage V an ignition coil designed to operate within a given temperature range and having an inductance L and a coil circuit resistance R having a positive temperature coefiicient, a semiconductor switch connected between the DC voltage source and the ignition coil, a control circuit connected to the DC voltage source and including a control input connected to the engine contact breaker system and an output connected to a control electrode of the semiconductor switch, said control circuit comprising a pulse generator including a resistancecapacitance network having a time constant RC and a voltage reference element, said pulse generator being responsive to a control signal to apply to the semiconductor switch a control pulse the duration of which is determined by the resistance capacitance network and the voltage V of said reference element so that a given energy is stored in the ignition coil and is released at the occurrence of the trailing edge of said control pulse, the components of the ignition system being chosen so that the time duration of the control pulse is substantially proportional to RC Ln V /V V to whereby the ignition coil energy has one value at the high temperature end and a substantially greater value at the low end of the temperature range.

po-loso 5/69) Inventor(s) Patent No. 3841288 Dated Aart Gerrit Korteling ctober 15, 1974 It is certified that error appears in the aboveidentifiedpatent and that said Letters Patent are hereby corrected as shown below:

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IN THE SPECIFICATIONS cancel ".Specifi";

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IN THE CLAIMS change "And" to "an" insert -by;

cancel "is" (1st occurrence) 7 Arrest:

RUTH c MASON .4 [testing Officer Signed and Scaled this twenty-ninth Day Of July 1975 C. MARSHALL DANN (vmmissimu'r oj'larenrs and Trademarks 

1. An ignition system for an internal combustion engine comprising, a source of DC voltage, and ignition coil having a positive temperature coefficient of resistance, a semiconductor switching device connected in series with the coil across the terminals of the DC voltage source, a control pulse generator connected to said DC supply terminals and having an output terminal coupled to a control electrode of the switching device for supplying thereto a switching control pulse for periodically switching the switching device on and off, means including the engine contact-breaker for supplying a control signal to said control pulse generator in timed relation to the engine, said pulse generator being responsive to the control signal to generate said control pulse and including an RC network that determines the time duration of the control pulse, a first voltage reference element connected in said pulse generator so as to cooperate with the RC network and the supply voltage to determine the time duration of the control pulse whereby the time duration of the control pulse varies inversely to the DC supply voltage VB, the value VR of the reference voltage of the reference element being chosen relative to the value of the DC supply voltage so that, in cooperation with the RC time constant of the RC network, the time duration of the control pulse is substantially proportional to RC 1n VB/VB - VR and varies over the non-linear portion of the exponential curve.
 2. A ignition system as claimed in claim 1, characterized in that the RC time constant is equal to the time constant L/RL of the ignition coil circuit, where L is the inductance of the ingition coil and RL is the resistance in the ignition coil circuit.
 3. An ignition system as claimed in claim 1 wherein the control pulse generator comprises a Schmitt trigger which includes a first transistor and a second transistor, said RC network comprising the series combination of a resistor and a capacitor connected between the supply terminals of the Schmitt trigger, means connecting the base of the first transistor to the junction point of the resistor and the capacitor, and the collector circuit of the second transistor, which is resistively coupled to the first transistor, delivers the control pulse to the switching device, and means connecting the voltage reference element in the common emitter circuit of the two transistors.
 4. An ignition system as claimed in claim 1 wherein the control pulse generator comprises a monostable multivibrator circuit in which the resistance-capacitance network is is connected so that the charging voltage of the capacitor C is equal to the voltage of the first voltage reference element, and a discharge circuit for the capacitor comprising a resistor R conncected in series with a second voltage reference element, the reference voltages of the two reference elements being equal.
 5. An ignition system as claimed in claim 1 wherein the control pulse generator includes a transistor connected in common emitter configuration the collector circuit of which includes a collector resistor connected in series with the voltage reference element and the base circuit of which includes said RC network comprising a series combination of a base resistor and a capacitor, which determines the RC time constant, and means coupling the transistor collector circuit to the switching device for delivering the control pulse thereto upon reception of a control signal at the control pulse generator input, said control pulse generator input being formed by the junction point of the base resistor and the capacitor, the base resistor being equal to the collector resistor multiplied by the current amplification factor of the transistor.
 6. An ignition system as claimed in claim 1 wherein said pulse generator comprises a transistor with the voltage reference element connected in the collector circuit and the RC netork connected in the base circuit.
 7. An ignition system as claimed in claim 1 wherein said pulse generator comprises a transistor with its collector coupled to the control electrode of the switching device, said RC network comprising a resistor and capacitor serially connected across the DC supply terminals and the voltage reference element being connected between the junction of the resistor and capacitor and the base of the transistor, and a diode connected between the control electrode of the switching device and said junction and poled to conduct current towards the junction.
 8. An ignition system as claimed in claim 1 wherein said pulse generator comprises a transistor with its collector coupled to the control electrode of the switching device via a second voltage reference element, said RC network comprising a resistor and capacitor serially connected across the DC supply terminals and the first voltage reference element being connected in the emitter circuit of the transistor.
 9. An ignition system as claimed in claim 1 wherein said pulse generator comprises first and second transistors connected to form a monostable multivibrator circuit with the capacitor of the RC network connected between the collector of the first transistor and the base of the second transistor, said first voltage reference element being connected to the collector of the first transistor so that the capacitor voltage is determined by the voltage of the first reference element, and a second voltage reference element coupled to the base circuit of the second transistor and as a part of a discharge circuit for the capacitor, the collector of the second transistor being coupled to the control electrode of the switching device for supplying said control pulse thereto.
 10. An ignition system for an internal combustion engine comprising, a source of DC voltage VB, an ignition coil designed to operate within a given temperature range and having an inductance L and a coil circuit resistance RL having a positive temperature coefficient, a semiconductor switch connected between the DC voltage source and the ignition coil, a control circuit connected to the DC voltage source and including a control input connected to the engine contact breaker system and an output connected to a control electrode of the semiconductor switch, said control circuit comprising a pulse generator including a resistance-capacitance network having a time constant RC and a voltage reference element, said pulse generator being responsive to a control signal to apply to the semiconductor switch a control pulse the duration of which is determined by the resistance capacitance network and the voltage VR of said reference element so that a given energy is stored in the ignition coil and is released at the occurrence of the trailing edge of said control pulse, the components of the ignition system being chosen so that the time duration of the control pulse is substantially proportional to RC Ln VB/VB - VR to counteract the exponential form of the ignition coil current, the ignition coil time constant L/RL and the RC time constant being chosen so that at the high end of said given temperature range the RC time constant is substantially equal to the time constant L/RL whereby the ignition coil energy has one value at the high temperature end and a substantially greater value at the low end of the temperature range. 